The present invention relates to a slippage prevention apparatus of a belt-drive continuously variable transmission for an automotive vehicle, and specifically to the improvement of techniques for optimizing a so-called speed-change hydraulic pressure to prevent slippage between a variable-width pulley and a drive belt incorporated in a belt-drive continuously variable transmission (CVT) through all engine operating conditions.
In recent years, there have been proposed and developed various belt-drive continuously variable transmissions (CVTs), which enable an actual transmission ratio to be steplessly adjusted or feedback-controlled toward a desired transmission ratio. As is generally known, in case of a lack of the speed-change hydraulic pressure with respect to the magnitude of input torque transferred to the input shaft of a belt-drive continuously variable transmission, there is an increased tendency for undesired slippage (frictional losses or power losses) between a variable-width pulley and a drive belt to occur. This causes drive-belt wear, thereby reducing the durability of the drive belt. On the contrary, excessively building up the speed-change hydraulic pressure with respect to the magnitude of input torque transferred to the CVT, causes a wasteful increase in the load on an engine-driven oil pump. For the reasons discussed above, it is desirable to keep the speed-change hydraulic pressure to just below a critical pressure level where a slippage between the variable-width pulley and the drive belt would start to develop. Actually, it is difficult to hold the speed-change hydraulic pressure to just below the critical pressure level for slippage prevention, because of various factors, that is, differences in quality of hydraulic parts used in a hydraulic modulator of the CVT, drive-belt wear, input-torque fluctuations, disturbance torque, a delay in response of the speed-change hydraulic pressure to a rapid change in input torque, and the like. To effectively prevent undesirable slippage irrespective of the presence or absence of the previously-noted factors, Japanese Patent Provisional Publication No. 9-53695 (hereinafter is referred to as xe2x80x9cJP9-53695xe2x80x9d, corresponding to U.S. Pat. No. 5,853,347, issued to Takashi Aoki et al. on Dec. 29, 1998) teaches the increase in a margin of the speed-change hydraulic pressure. On the other hand, Japanese Patent Provisional Publication No. 8-258595 (hereinafter is referred to as xe2x80x9cJP8-258595xe2x80x9d) teaches the limitation or suppression of a time rate of change in input torque transferred to the CVT to avoid a rapid change in the CVT input torque, occurring due to a sudden driver""s accelerator pedal depression.
In the CVT disclosed in JP9-53695, the margin of the speed-change hydraulic pressure is constantly set or maintained at a high level, and thus the load on the engine-driven oil pump tends to increase. This deteriorates fuel economy and lowers the vehicle driveability. On the other hand, in the control apparatus for the CVT and internal combustion engine, disclosed in JP8-258595, when the accelerator pedal is greatly quickly depressed owing to the driver""s wishes (i.e., quick accelerating operation), it is impossible to timely precisely increase the CVT input torque responsively to the rapid accelerator pedal""s depression, because of the limitation or suppression of the time rate of change in CVT input torque. As a result, it is impossible to provide a better response to a quick acceleration (a sudden depression of the accelerator pedal). This lowers the acceleration performance of the vehicle. The inventor of the present invention directs attention to the fact that slippage of the belt-drive CVT becomes an issue in particular when quickly accelerating the vehicle.
Accordingly, it is an object of the invention to provide a slippage prevention device of a belt-drive continuously variable transmission for an automotive vehicle, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a slippage prevention apparatus of a belt-drive continuously variable transmission for an automotive vehicle, which is capable of improving fuel economy, and enhancing the vehicle driveability, and acceleration performance by optimizing a so-called speed-change hydraulic pressure depending on a frequency of quick accelerating operations.
In order to accomplish the aforementioned and other objects of the present invention, a slippage prevention apparatus of a belt-drive continuously variable transmission for an automotive vehicle whose transmission ratio is controlled by a speed-change hydraulic pressure brought closer to a desired speed-change hydraulic pressure, comprises a quick-acceleration-frequency decision section that determines whether a frequency of quick accelerating operations is low or high, and a desired hydraulic pressure decision section that sets the desired speed-change hydraulic pressure used during an operating mode that the frequency of quick accelerating operations is low to a relatively lower pressure level than the desired speed-change hydraulic pressure used during an operating mode that the frequency of quick accelerating operations is high.
According to another aspect of the invention, a slippage prevention apparatus of a belt-drive continuously variable transmission for an automotive vehicle whose transmission ratio is controlled by a speed-change hydraulic pressure brought closer to a desired speed-change hydraulic pressure, comprises a quick-acceleration-frequency decision means for determining whether a frequency of quick accelerating operations is low or high, and a desired hydraulic pressure decision means for setting the desired speed-change hydraulic pressure used during an operating mode that the frequency of quick accelerating operations is low to a relatively lower pressure level than the desired speed-change hydraulic pressure used during an operating mode that the frequency of quick accelerating operations is high.
According to a further aspect of the invention, a method of preventing slippage between a variable-width pulley and a drive belt of a belt-drive continuously variable transmission for an automotive vehicle whose transmission ratio is controlled by a speed-change hydraulic pressure brought closer to a desired speed-change hydraulic pressure, the method comprises determining whether a frequency of quick accelerating operations is low or high, setting a low-pressure enabling flag during an operating mode that the frequency of quick accelerating operations is low, resetting the low-pressure enabling flag during an operating mode that the frequency of quick accelerating operations is high, and setting the desired speed-change hydraulic pressure used during the operating mode that the low-pressure enabling flag is set to a relatively lower pressure level than the desired speed-change hydraulic pressure used during the operating mode that the low-pressure enabling flag is reset.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.